η3-Allylpalladium Complexes from Medium-Ring Cycloalkenes

The dimeric η3-allylpalladium chloride complexes formed from various cycloalkenes (C7−C13) and some methyl- and tert-butyl-substituted cycloalkenes have been characterized by 1H and 13C NMR spectroscopy and in selected cases by X-ray crystallography. The formation of syn and anti isomers in the larger ring systems is demonstrated, and complexes with a rearranged ring system are formed from tert-butylcyclodecene and tert-butylcyclododecene. The sesquiterpene, carophyllene, is shown to form an η3-allyl exocyclic complex, exclusively from the (E)-double bond. Cis and trans isomers, with respect to allyl group orientation in these halo-bridged dimers, have been identified by low-temperature (190K) NMR spectroscopy, and their interconversion (ΔG ⧧ ≈ 11 kcal/mol) is considered to involve a “cubic” η3-allylpalladium chloride tetramer.